Ion beam implanters are widely used in the process of doping of semiconductor wafers with a desired species of ions. An ion beam implanter generates an ion beam comprises of the desired species of positively charged ions. The ion beam impinges upon an exposed surface of a semiconductor wafer workpiece thereby "doping" or implanting the workpiece surface with desired ions. Some ion beam implanters utilize serial implantation wherein a single semiconductor wafer workpiece is positioned on a support in an implantation chamber. The support is oriented such that the workpiece is in the ion beam beam line and the ion beam is repetitively scanned over the workpiece to implant a desired dosage of ions. When implantation is complete, the workpiece is removed from the support and another workpiece is positioned on the support.
Another type of ion beam implanter uses a rotating, translating disk-shaped support on which workpieces are mounted. A plurality of semiconductor workpieces are mounted on the disk-shaped support. The support is supported in an implantation chamber of an end or implantation station of the ion beam implanter. The rotation and translation of the support allows each of the plurality of workpieces to be exposed to the ion beam during a production run.
Accuracy in both: a) the quantity of ions implanted in a semiconductor wafer workpiece during the implantation process; and b) the implantation depth of ion implantation in the workpiece surface are of critical importance in producing an acceptable end product. The allowable tolerances on implantation depth and total ion implantation quantity or dose in the manufacturing of many semiconductor devices are now at the 1% level in many applications.
Ion implantation depth of workpieces in an ion beam implanter is directly dependent on the energy of the ion beam. Therefore, accuracy in achieving desired implantation depth requires accurate control, measurement and monitoring of the energy of the ion beam.
Prior art high energy ion implanters control ion beam energy using a final energy magnet (FEM). The control of ion beam energy using an FEM assumes the energy of the ion beam can be selected by choosing the strength of the magnetic field needed to "bend" or cause desired species of ions comprising the ion beam to move in a predetermined accurate path toward workpieces supported on a support within the implantation chamber. The ion beam is directed though the FEM. The magnetic field causes the ions comprising the ion beam to travel in an accurate path. The strength of the magnetic field is adjusted such that desired species of ions having a particular momentum follow a curvilinear path to the implantation chamber where the workpieces are disposed.
Unfortunately, the accuracy of the FEM method of computing ion beam energy has a serious drawback in that the magnitude of bending of the ion beam is a function of the entry angle of the ions into the magnetic field of the FEM. Even a slight difference of entry angles (2-3 degrees) of the ions when entering the FEM magnetic field has a significant impact on the magnitude of the ion beam bending. Numerical simulations and empirical tests have shown that the FEM energy readback can be incorrect by up to +/-10% of the desired energy of the ion beam under certain conditions.
What is needed is a more accurate ion beam energy measurement apparatus for an ion implanter. What is also needed is such an energy measurement apparatus that is relatively inexpensive, durable and that provides for rapid, real time updating of beam energy. What is further needed is an energy measurement apparatus that can be retrofit for use on current generation ion beam implanters without requiring extensive modification of an implanter.